This invention relates to a coated material such as a tool material in which a layer of high hardness is formed to a surface of a substrate. This invention also relates to an X-ray exposure mask used for X-ray lithography, for example.
Heretofore, working life of tools has been extended by forming a layer of high hardness comprising boron nitride (BN) or silicon nitride (Si.sub.3 N.sub.4) to the surface of a substrate of the tool material by means of thermal CVD process, plasma CVD process, light CVD process, or the like.
A layer comprising boron nitride shows a high hardness, but is poor in its toughness. Accordingly, if material coated with the layer is used in a tool, for example, there occurs a problem that the blade tip tends to be lost by chipping. On the other hand, a layer comprising silicon nitride has a good toughness, but is inferior in its hardness as compared with the boron nitride. Accordingly, its use leads to a drawback in view of the working life, etc. of the tool.
Furthermore, such a layer comprising boron nitride or silicon nitride as described above is also employed as an X-ray exposure mask, as will be described hereinafter.
FIGS. 5A through 5F show one example of the manufacturing steps for constructing an X-ray exposure mask. The manufacturing steps to construct an X-ray exposure mask are as follows: A mask support 32 comprising, for example, a silicon single crystal substrate is prepared (FIG. 5A). An X-ray permeable support layer 34 comprising a thin film of boron nitride (BN) or a thin film of silicon nitride (SiNx) is formed on the support 32 by means of a CVD process or a PVD process (FIG. 5B). Then, an X-ray absorbent layer 36 made of, for example, Au, Ta, W, or the like is formed on the layer 34 by means of CVD process, PVD process, or the like (FIG. 5C). In this way, a mask (mask blank) 37 which is not yet subjected to a process such as patterning, etc. can be obtained. Subsequently, for example, after forming a resist pattern 38 over the X-ray absorbent layer 36, (FIG. 5D), the X-ray absorbent layer 36 is patterned by means of ion etching or the like (FIG. 5E). Finally, a window-perforation is applied to the mask support 32, using the support layer 34 as an etching stopper layer by means of wet etching, etc. to obtain a finally fabricated X-ray exposure mask 40 (FIG. 5F).
In the X-ray exposure mask, conventionally, a thin film of boron nitride or a thin film of silicon nitride has generally been used for the X-ray permeable support layer 34, as described above. However, since the thin film of boron nitride is poor in toughness although having high hardness, it has a drawback of easy cracking thus requiring utmost care for the handling thereof in case of using it as the support layer 34. On the other hand, the thin film of silicon nitride has an extremely excellent toughness as compared with the thin film of boron nitride, but is poor in its hardness. Therefore, in the case where silicon nitride is used for the support layer 34 and the window-perforation is applied to the mask support 32, it involves a drawback of warping. Therefore, it is impossible to conduct exact pattern transfer.